Trials Bikes (needing advice)

First, I apologize for putting this in the general discussion, but I could find another subsection that would work for a trials bike question.

I am interested in starting to build a trials bike. Unfortunately, my bike knowledge is extremely limited, especially when it comes to such specialized bikes.

While I plan on using this bike myself, it would be for my fiance, who has expressed extreme interest in trials. He was an avid BMXer when he was younger and, after attending Interbike, is dying to get back on a bike. He has a decent BMX bike right now, but I know he wants a trials bike more than anything.

Though it is tempting to buy one of the Inspired trails bikes, I know he would much prefer a custom built bike (as that is how almost all of his BMX bikes have been).

I plan to show him items and get his opinions on them, then surprise him with a complete (or mostly complete) bike (and the receipts, just in case). I just have absolutely no clue where to start!

Brakes are usually magura hydraulic rim (discs would catch when you slip off a log).
Wheels are 20, 24 or 26"
There seem to be lots of small brands building trials bikes so building yourself would be an expensive option. Is there anything in particular that the built-up trials-bikes lack?